Regulators and health experts are particularly concerned about a device called the Juul, which packs the same nicotine content per pod as a pack of cigarettes.

Smoking kills. No other habit has been so strongly tied to death.

In addition to inhaling burned tobacco and tar, smokers breathe in toxic metals like cadmium and beryllium, as well as metallic elements like nickel and chromium – all of which accumulate naturally in the leaves of the tobacco plant.

Very few studies, however, look at how vaping affects the body and brain. Even fewer specifically examine the Juul, a popular device that packs as much nicotine in each of its pods as a standard pack of cigarettes.

But a handful of studies published in the past few months have begun to illuminate some of the potential health effects tied to vaping. They are troubling.

With that in mind, the Food and Drug Administration outlined a new policy on Thursday morning designed to eventually curb the sale of e-cigs and reign in their appeal to young people.

Most recently, researchers at the Stanford University School of Medicine surveyed young people who vaped and found that those who said they used Juuls vaped more frequently than those who used other brands. The participants appeared to be insufficiently aware of how addictive the devices could be.

Most e-cigs contain toxic metals, and using them may increase the risk of a heart attack

Researchers took a look at the compounds in several popular brands of e-cigs (not the Juul) this spring and found some of the same toxic metals (such as lead) inside the device that they would normally find in conventional cigarettes. For another study published around the same time, researchers concluded that at least some of those toxins appeared to be making their way through vapers’ bodies, as evidenced by a urine analysis they ran on nearly 100 study participants.

In another study published this summer, scientists concluded that there was substantial evidence tying daily e-cig use to an increased risk of heart attack. And this week, a small study in rats suggested that vaping could have a negative effect on wound healing that’s similar to the effect of regular cigarettes.

In addition to these findings, of course, is a well-established body of evidence about the harms of nicotine. The highly addictive substance can have dramatic impacts on the developing brains of young adults.

Brain-imaging studies of adolescents who begin smoking traditional cigarettes (not e-cigs) at a young age suggest that those people have markedly reduced activity in the prefrontal cortex and perform less well on tasks related to memory and attention, compared with people who don’t smoke. Those consequences are believed to be a result of the nicotine in the cigarettes rather than other ingredients.

Nicholas Chadi, a clinical pediatrics fellow at Boston Children’s Hospital, spoke about the Juul at the American Society of Addiction Medicine’s annual conference this spring. He said these observed brain changes were also linked to increased sensitivity to other drugs as well as greater impulsivity. He described some anecdotal effects of nicotine vaping that he’d seen among teens in and around his hospital.

“After only a few months of using nicotine,” Chadi said, the teens “describe cravings, sometimes intense ones.” He continued: “Sometimes they also lose their hopes of being able to quit. And interestingly, they show less severe symptoms of withdrawal than adults, but they start to show them earlier on. After only a few hundred cigarettes – or whatever the equivalent amount of vaping pods – some start showing irritability or shakiness when they stop.”

A new survey suggests that teens who use Juul e-cigs aren’t aware of these risks

Yet very little research about e-cigs has homed in on the Juul specifically.

So for a study published this week, researchers from the Stanford University School of Medicine surveyed young people who vaped and asked them whether they used the Juul or another e-cigarette.

Their results can be found in a widely accessible version of the Journal of the American Medical Association called JAMA Open. Based on a sample of 445 high-school students whose average age was 19, the researchers observed that teens who used the Juul tended to say they vaped more frequently than those who used other devices. Juul users also appeared to be less aware of how addictive the devices could be compared with teens who vaped other e-cigs.

“I was surprised and concerned that so many youths were using Juul more frequently than other products,” Bonnie Halpern-Felsher, a professor of pediatrics who was a lead author of the study, said in a statement.

“We need to help them understand the risks of addiction,” she added. “This is not a combustible cigarette, but it still contains an enormous amount of nicotine – at least as much as a pack of cigarettes.”